Author: David Peacham
Date: 2004-09-28 19:11
Re Helium, pitch, etc.
The pitch we hear is determined by frequency. The way we produce different notes on woodwind and brass instruments is not to control the frequency directly. What we do is alter the effective length of the tube, and hence the wavelength. But the speed of sound is equal to frequency times wavelength. So long as the speed of sound remains constant, shorter wavelength means higher frequency/pitch, longer wavelength means lower frequency/pitch.
If you change the composition or temperature of the air inside the instrument, the speed of sound changes. So if you play in helium, the speed of sound is much faster, and the same wavelength gives a higher pitch. Conversely, a dense gas such as krypton would lower the pitch. Similarly, a rise in temperature causes the speed of sound to increase, which is why pitch rises as the instrument gets warm.
Antony Pay claims that you should not breathe in immediately before an solo entry, but several seconds before. His argument, if I remember correctly, is that if you play with fresh air then the carbon dioxide content of your breath will be lower than with stale air. Carbon dioxide is relatively heavy, so your solo entry will start sharp.
This is true of any woodwind or brass instrument, and of the organ. It is not true of string instruments, nor of free-reed instruments like the harmonica and harmonium, since these instruments do not rely on a column of air.
Also, oddly enough, it is not true of the voice. If you inhale helium, the pitch of your voice does not change. Yes, I know your voice will sound squeaky. But what has changed is not the pitch, but the timbre.
You don't believe me:
http://www.newscientist.com/lastword/article.jsp?id=lw802
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If there are so many people on this board unwilling or unable to have a civil and balanced discussion about important issues, then I shan't bother to post here any more.
To the great relief of many of you, no doubt.
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